Most of the answers below do not address the framerate issue in the question. Please don't just suggest a tool, but demonstrate how that tool may be used to correctly join multiple video files with differing framerates.
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Ryan ThompsonMay 27 '11 at 23:13

@adam-matan - I'm intrigued - instead of using multiple tools, have you tried rotating clips in OpenShot? You can also change the speed to the video clip to either "1/2 or 2x" depending upon the framerate (right click the click and choose properties)
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fossfreedom♦May 28 '11 at 10:00

4 Answers
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I'm not clear if you want a command line type answer, but given that you said that there are different videos and framerates and you want to do simple edits, then I would use a good video editor such as OpenShot or KdenLive to amalgamate all the videos on one track and export as one video format.

May be this question deserves more an answer for a procedure other than a single shot command or single gui process.

Transcoding the videos may be a solution for you

First of all you should make all your videos to be the same frame rate, then your own command will result in what you wish.

There are several suggestions for transcoding tools in this site but I am going to point you to my answer, if you don't mind of course, then, take a look at this: How can I maximum compress video files?

The interesting of this tool (The Mobile Media Converter) is that if you wish to have the command line for the transcodification and modify it for making a batch command line process, you can enter the "Advanced" section which will offer you different parameters to be set up manually and there will also be the FFMPEG command that you can drop in a terminal in order to transcode.

After that, using your command line will result in concatenating video files if it is proved by you to work that way with same frame rate video files.

Have you given mjpegtools a shot? It has been a while since I have messed around with video editing/conversion, but when I last used mjpegtools it was a CLI utility. Still, very powerful and easy to use. Mjpegtools can be found at sourceforge.net.